The present invention relates generally to industrial process field devices, and more particularly to a hybrid power module for powering a wireless industrial process field device.
The term “field device” covers a broad range of process management devices that measure and control parameters such as pressure, temperature, and flow rate. Many field devices are transmitters which act as communication relays between a transducer for sensing or actuating an industrial process variable, and a remote control or monitoring device such as a computer in a control room. The output signal of a sensor, for example, is generally insufficient to communicate effectively with a remote control or monitoring device. A transmitter bridges this gap by receiving communication from the sensor, converting this signal to a form more effective for longer distance communication (for example, a modulated 4-20 mA current loop signal, or a wireless protocol signal), and transmitting the converted signal to the remote control or monitoring device.
Field devices are used to monitor and control a variety of parameters of industrial processes, including pressure, temperature, viscosity, and flow rate. Other field devices actuate valves, pumps, and other hardware of industrial processes. Each field device typically comprises a sealed enclosure containing actuators and/or sensors, electronics for receiving and processing sensor and control signals, and electronics for transmitting processed sensor signals so that each field device and industrial process parameter may be monitored remotely. Large scale industrial manufacturing facilities typically employ many field devices distributed across a wide area. These field devices usually communicate with a common control or monitoring device, allowing industrial processes to be centrally monitored and controlled.
Field devices increasingly use wireless transceivers to communicate with centralized control or monitoring systems. Wireless devices extend the reach of control or process monitoring systems beyond that of wired devices to locations where wiring may be difficult and expensive to provide. In some cases wireless field devices may be powered by direct electrical connection to power utilities such as 120V AC utilities, or powered data. More often, however, power utilities are not located nearby or cannot readily be installed in hazardous locations where instrumentation and transducers must operate. Accordingly, field devices are often locally powered by power sources with limited capacity, either stored, as in the case of a long-life battery, or produced, as in the case of a solar panel. Batteries are expected to last more than five years and preferably last as long as the life of the product. Because local power sources have limited capacities, the use of lower power electronics and RF radios is frequently essential for many wireless field devices.
Many field device designs enclose an attached battery under a cover of the sealed enclosure of the field device. Other field devices utilize power from external sources such as solar panels, energy harvesters such as vibrational or thermo-electric scavengers, or a nearby utility grid connection. Each method of powering a wireless field device conventionally requires a different wiring terminal interface. Field devices which run partly or entirely on battery power typically incorporate terminal blocks which provide contact points to an attached battery. Field devices which run on grid power, by contrast, include terminal blocks which provide wired connections for grid power (typically via screw terminals), and which condition grid power for use by the field device. Terminal blocks are often removable, allowing a single field device to be configured for different power sources by swapping in one or another source-specific terminal block. Solar panels, vibrational energy scavenging systems, and other types of local power modules may all use different terminal blocks.
Wireless transmitter field devices broadcast periodic signals corresponding to sensed parameters. Battery-powered transmitters are typically expected operate for five or more years between battery replacements. Depending on the application, existing systems can operate for this period of time while transmitting as often as once every four seconds. Faster update rates are desirable for many industrial applications, but necessitate greater power draw which significantly reduces battery life.
Energy harvesting systems such as solar panels and vibrational or thermoelectric scavengers produce power highly dependent on location and application. Vibrational scavengers can be highly efficient energy sources in areas with high amplitude continuous vibration, for instance, but may not be practical or sufficient in areas with low amplitude or intermittent vibration. Furthermore, while batteries and supercapacitors ordinarily continue to provide power while discharging, energy harvesting systems may experience unpredictable drops in power production, resulting in fluctuating levels of power depending on environmental conditions. Solar panels, for instance, produce no power in the dark, and vibrational scavengers produce no power when attached structures (e.g. motors) are still.